Batteries can store lots of energy but have low power, taking a long time to charge or discharge. Conventional capacitors have enormous power but can usually only store tiny amounts of energy. Supercapacitors offer a unique combination of high power and high energy. When a supercapacitor is charged, there is no chemical reaction, rather energy is stored as a charge or concentration of electrons on the surface of a material. Supercapacitors can therefore charge and discharge at high rates since only charge transfer is involved, and can undergo a large number of cycles without any apparent degradation (e.g. >100,000 cycles).
Supercapacitors, also known as ultracapacitors, or electrochemical double layer capacitors (EDLC) use a double layer of high surface area carbon separated by an exceedingly thin insulator to achieve high capacitance. In a paper entitled “Novel Supercapacitors”, p 68, Proceedings of the 40th Power Sources Conference, Cherry Hill, USA, Jun. 10-13, 2002 (hereinafter referred to as the “2002 paper”), one of the present inventors, inter alia, disclosed a new type of supercapacitor where, in addition to the double layer capacitance, additional capacitance or “pseudo-capacitance” was obtained using nitrogen-containing carbon polymers where complexes had been incorporated at certain nitrogen sites, migration of solution species towards and away from these sites giving rise to this pseudo-capacitance.
In the 2002 paper, polyacrylonirile (PAN) was used as a cheap and readily available polymer. Pyrolysis of PAN leads to increasingly complex, conjugated and cyclised molecules containing nitrogen in pyridine-like rings, but it was found that N content decreases with increased heating, as illustrated in FIG. 3 below. Steps were therefore taken to obtain a pyrolysed PAN with a sufficient N content but also with a suitably high surface area, open, porous structure that would permit complex formation and ion species migration. The pyrolysed PAN was then combined with lithium sulphite, Li2SO3, with the aim of subsequently forming N:sulphur trioxide SO3 complexes at the nitrogen sites. The 2002 paper describes lithium sulphite as being incorporated in the pyrolysed polymer and then provided as an electrode in a cell where, upon charging, the lithium sulphite was electrolysed to form sulphur trioxide, which subsequently complexed with nitrogen in the pyrolysed PAN. Supercapacitor cells made from those modified electrodes were found to have increased capacitance as compared with unmodified PAN electrodes (i.e. without complexed N sites).
One of the authors of the 2002 paper also filed two patent applications published as GB2266179 and GB2335073, respectively. GB2266179 teaches secondary cells in which the cathode (positive electrode) composition consists of a solid complex of sulphur trioxide and a polymeric tertiary amine (the complex being formed by a previous chemical reaction) and the anode is lithium. GB2335073 describes GB2266179 as disclosing electrochemical cells with low energy densities due to the high ratios of carbon to nitrogen functionality. GB2335073 itself is directed to improved secondary cells with similar light metal anodes and similar cathodes of N—C polymers complexed with sulphur trioxide except that these polymers have conjugated imine functionality, preferably formed by thermally restructuring nitrile containing polymers. GB2335073 teaches that charging of the secondary cells causes lithium to be electroplated at the anode and discharging causes recombination of lithium with the SO3 complexes: hence, it states that SO3 acts not only as polymer dopant but also as sites for the reversible redox electrochemistry of Li2SO3.
Journal of the Chemical Society, pages 2245-8, (1928), describes a method for synthesising lithium sulphite in which a mixture of hydrogen and sulphur dioxide is passed through lithium carbonate solution until effervescence ceases, and the flask is then heated while passing through dry hydrogen (to prevent oxidation) until anhydrous lithium sulphite is produced. Since sulphur dioxide is a corrosive gas, and hydrogen is a flammable gas, this process is not ideal.